1. Field of the Invention
The present invention relates to a semiconductor laser diode and a process for fabrication of the same, and more particularly to a buried heterostructure laser diode containing a current blocking layer and a process for fabrication of the same.
2. Description of the Prior Art
A buried-structure laser diode (BH-LD) attracted general attention because of its stability in oscillation modes and low oscillation thresholds accordingly vigorous research and development thereof is progressing throughout the world. The BH-LD having a InGaAsP/InP DH (Double Hetero) structure is expected to operate as a favorable light source for an optical fiber communication system because its oscillation wavelengths correspond to a 1.2-1.7 .mu.m band which is a low transmission loss band of quartz series optical fibers.
Normally, the BH-LD requires two different epitaxial growth steps The first LPE (Liquid Phase Epitaxy) step is for forming a DH structure and the second LPE step forms a buried structure. Despite its excellent characteristics, however, such a BH-LD is not desirable because its fabrication yield is lowered due to the addition of an etching step for a DH structure wafer and a LPE step for burying. In addition, the degradation of reliability and operation characteristics caused by placing a DH structure wafer including a partly exposed active region under a high-temperature condition during the LPE step for burying, is a problem which must be further investigated.
Heretofore, K. Kishino et al discovered that a BH-LD having a InGaAsP/InP DH structure can be realized through a single LPE step. The Kishino et al. invention is referred to as a "Mesa Substrate Buried Heterostructure Diode (MSB-LD)," and the results of trial fabrication is reported in IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-16, No. 2, FEBRUARY 1980, pages 160-164.
The MSB-LD was expected to improve reliability by eliminating the step of subjecting an exposed active region to a high temperature and to enhance a yield and lower cost by greatly shortening the process. However, since it was difficult to effectively form current confinining means, a sufficiently low oscillation threshold could not be obtained, and the realized oscillation threshold was as high as about 100 mA. More particularly, Kishino et al discovered that in a MSB-LD, if the cross-sectional configuration of the mesa stripe formed on the substrate, that is the height of the mesa and the slope angle of the mesa side face were appropriately selected, an InGaAsP layer could be grown on top of the mesa stripe separately from that of the mesa bottom. However, according to the propsed process, although an active region of 4-5 .mu.m or more in width can be repeatedly obtained, it was difficult to form an active, region having a width of about 2 .mu.m i.e., an active region favorable for stabilizing an oscillation mode and realizing a low oscillation threshold.